1. Field of the Invention
This invention relates to aircraft navigation equipment, and more particularly to a manually operable descent calculator for determining the optimal fuel efficient point at which to commence a descent.
2. Description of the Prior Art
Significant fuel ecomonies can be achieved when an aircraft follows a proper descent profile from cruise altitude on approach to landing. Conversely, excess and unnecessary amounts of fuel will be expended if the aircraft begins its descent either too soon or too late. Typically, the pilot will be instructed to descend from a cruise altitude of perhaps 35,000 feet to a target altitude of perhaps 13,000 feet, at a specified distance from the runway. If the pilot begins to descend at too great a distance from the airport, the descent will be unnecessarily shallow and result in greater fuel consumption than on an optimum descent. On the other hand, if the pilot doesn't begin the descent until after the optimal point has been passed, the descent will be too steep for minimum fuel consumption, and again fuel will be wasted.
Calculations have been performed in the past to determine the optimal descent profile in terms of fuel economy for a given aircraft speed, altitude, target altitude, and target distance from the runway. The problem is to present this information to the pilot in a useable format under actual flight conditions, in which the pilot's attention may be diverted by many other factors when beginning a descent to land. Any instrument for informing the pilot when to begin a descent should be sufficiently flexible to adapt to different aircraft parameters such as speed and altitude, easy and quick to operate, and display a clear result. While a digital computer could be programed to provide the desired descent profile information, such equipment is fairly expensive and would occupy valuable space in the aircraft console.
One prior art attempt to provide a pilot with useable information on when to begin a descent is disclosed in U.S. Pat. No. 4,011,987, issued Mar. 15, 1977 to Frank L. Cheek. The patent discloses an aircraft descent profile calculator with three members defining outer, intermediate and inner annular dial faces. The intermediate dial face has aircraft altitude indicia displayed on it which increase in prescribed altitude increments in a first direction around the intermediate dial face. The inner and outer dial faces have aircraft DME (Distance Measuring Equipment station) distance displays which increase in prescribed distance increments in opposite directions around the dial faces. Approaching DME distance indicia are recorded on the outer disk member, while leaving DME distance indicia are recorded on the inner disk member. The relationship between the altitude indicia in adjacent spaces on the intermediate disk member compared to the DME distance indicia on the inner and outer disk members is selected to provide a known descent profile. A DME distance for beginning a descent from an aircraft's actual altitude to a target altitude and DME distance is obtained by aligning the target altitude on the intermediate disk with the target distance on either the inner or outer disk and comparing the actual altitude with the corresponding distance on the inner or outer disk.
The Cheek patent is based on a fixed ratio linear descent profile of 1000 feet altitude loss per 3 or 4 nautical miles, depending upon the rate of descent selected. This, however, is only an approximation of actual descent performance; the optimal ratio for greatest fuel economy varies substantially with air speed, wind and aircraft weight difference. The Cheek patent is subject to improvement because it does not provide for the above factors.
Another aircraft flight path calculator is disclosed in U.S. Pat. No. 3,929,279, issued Dec. 30, 1975 to Dibley. In this device an inner dial is divided into DME distance increments, while an outer dial has two altitude scales, one for the altitude range from 10,000 through 40,000 feet, and the other from ground zero through 10,000 feet. The two altitude scales have different proportions, corresponding to different air speeds above and below 10,000 feet. The upper altitude scale is also extended down to ground zero to accommodate the possibility of an approach without a deceleration interval. Around the periphery of the outer element are displayed two different scales relating rate of descent to actual ground speed, one scale corresponding to a descent gradient of 400 feet per mile at a ground speed of 340 knots, and the other scale corresponding to a descent gradient of 300 feet per mile at a ground speed of 250 knots. While the patent specification is not explicit concerning the detailed operation of the calculator, the pilot apparently aligns the target distance with the target altitude, and then refers to his actual altitude to obtain distance for commencing a descent. The pilot then apparently refers to the descent rate/ground speed scales to determine the rate of descent he should follow. While the latter scales introduce aircraft weight and wind speed as factors in the descent profile to be followed, in the sense that weight and wind will effect ground speed and thus the rate of descent the pilot is to follow, the initial point at which the aircraft begins to descend does not change to correspond to differing weight and wind conditions. This in turn leads to greater than necessary fuel consumption, since for optimal fuel consumption of the aircraft should begin to descend at a greater distance for tailwinds or heavy loadings, and at a lesser distance for headwinds or light loadings. Furthermore, the Dibley patent does not provide for a change of speed other than at 10,000 feet altitude, nor does it take into account the fact that aircraft speed is normally given as mach speed above roughly 30,000 feet, and as indicated air speed below roughly 30,000 feet. In addition, the Dibley device assumes the use of power management to provide a descent rate, which is a more difficult procedure than an idle descent.
Thus, there is still a need for an aircraft descent calculator which is capable of taking into account and adjusting to different conditions or aircraft weight, wind, aircraft speed and altitude, and without unduly complicating the device.